1. Field of the Invention
The present invention relates to a film feed device which feeds a film by using a motor as a drive source.
2. Related Background Art
FIG. 1 shows a construction of a film wind-up unit.
A drive force of a motor 1 is transmitted to a spool 4 which is in union with a gear 3 through gears 2 to rotate the spool 4.
The spool 4 is a member to wind up the taken-up film and the film is wound up on the spool 4 by the drive of the motor 1.
The film rotates a sprocket 9 while it is taken up.
The rotation of the sprocket 9 causes the rotation of a disk 7 which rotates with a gear 6 through the gears 5. A plurality of grooves 7a are formed in the disk 7 at a constant pitch, and a photo-interrupter 8 detects the grooves 7a and outputs a pulse signal for each unit of movement. The rotation of the disk 7 is detected by this output signal.
Since the rotation of the disk 7 is linked to the rotation of the sprocket 9, a feed velocity of the film is detected by detecting the rotation speed of the disk 7.
When the film is fed, since the taken-up film is wound on the spool 4, a diameter of the spool 4 when the 35th or 36th frame of the film is wound is greater than that when the first or second frame is wound. Assuming that the rotating speed of the spool 4 is constant without regard to the number of frames of the film, the wind-up speed (feed velocity) is larger when the diameter is greater.
Further, when a voltage of a power supply battery for driving the motor drops, the rotation speed of the motor, that is the feed velocity of the film decreases accordingly. As a result, the film feed velocity changes between when a new battery is used and when an old battery is used, or between when the camera is used at a high temperature and when the camera is used at a low temperature.
Accordingly, the amount of feed of the film is not constant because the feed velocity of the film is not constant.
In the light of the above, it is required in the film feed device to shorten the feed time by quickly feeding the film and stopping it at a constant position.
In a prior art film feed device, the following method has been proposed to stop the feed of the film.
In a first method, a duty factor (a ratio of a film feed velocity before the duty drive and a film feed velocity after the drive) of the duty feed for reducing the feed velocity ( moving velocity ) of the film is varied so that the feed velocity of the film at the time of start of short braking to stop the motor is always constant.
FIG. 2 shows a chart of a relationship between the film feed velocity and the film feed amount in the film feed device. In FIG. 2, Sd-Ss represents a duty drive section, and Ss-Se denotes a short brake section. A graph a represents a relation between the film feed velocity and the film feed amount when the film is stopped from a high speed feed status, and graph b represents a relation between the film feed velocity and the film feed amount when the film is stopped from a low speed feed status. In the graph a, a duty factor in the duty drive section is rendered large to rapidly decelerate between the film feed velocities Va and Vs while in the graph b the duty factor in the duty drive section is rendered small to slowly decelerate between the film feed velocities Vb and Vs.
In this manner the film feed velocity after the duty drive is rendered constant (Vs) and then the short brake is applied.
However, in this method, when the feed velocity is high, the duty drive time is long and hence the film feed time is long. Further, since the motor is stopped by the short brake, the overrun amount (a distance of film movement to the complete stop of the motor after the application of the brake Ss-Se) is large and the variation is large because of the inertia of the motor and the gears and the friction. A second method to stop the film is now explained.
In the second method, the time at which the brake is applied to the feed motor is changed in accordance with the film feed velocity and the motor is stopped by the short brake so that the film is always stopped at the constant position.
FIG. 3 shows a chart of a relation between the film feed velocity and the film feed amount in the present film feed device.
A graph a represents a relation between the film feed velocity and the film feed amount when the film is stopped from the high speed feed status, and a graph b shows a relation between the film feed velocity and the film feed amount when the film is stopped from the low speed feed status. In the graph a, the short brake is applied at a film displacement point Ssa while in the graph b the short brake is applied at the film displacement point Ssb which is larger than that of the graph a. In this case, since the higher the feed velocity is, the longer is the short brake applied and hence the larger is the overrun amount. Further, the variance is larger due to the inertia of the motor and the gears and the friction.